The invention relates to a method and a device with a control apparatus for starting an electric machine with a magnetically mounted rotor, in particular for rotating the spinning cup of an open-end spinning machine. The magnetic bearing has permanent magnets, an actuator system for controlling the position in the axial direction and two axial limitation bearings, by means of which the axial end positions of the rotor are determined.
Electric machines with magnetically mounted rotors are used, for example, as the drive of gas ultracentrifuges, turbomolecular pumps, tool spindles, compressors, blood pumps and flywheels. A magnetic bearing is advantageous in particular at high rotational speeds, as it has a significantly lower power loss compared to mechanical bearings such as roller or sliding bearings. A particular area of use is the drive of the spinning cup of open-end spinning machines. Rotational speeds of over 100,000 revolutions per minute are required here.
German Patent Publication DE 100 22 736 A1 discloses a magnetically mounted drive for a spinning rotor of an open-end spinning machine and describes the control and the structure of a magnetic bearing. The magnetic bearing shown here consists of two permanent magnetic rings in each case on each shaft end. A magnetic bearing with permanent magnets, a passive bearing is also referred to, is unstable in at least one axis. The cited prior art shows a radial bearing which is instable in the axial direction. For this reason, actuators are necessary, with the aid of which the bearing can be controlled in the axial direction. Coils or windings which may amplify or weaken the magnetic field of the permanent magnets are suitable as actuators. In order to prevent the bearing magnets impacting against one another if the actuator fails, or in the rest state when the machine is switched off, an arrangement of this type has two axial limitation bearings, also called safety bearings. The axial end positions of the rotor are determined by the limitation bearings. In the rest state, the rotor rests on one of the axial limitation bearings. When the machine is started up the rotor, through suitable activation of the actuator, has to lift off from its limitation bearing.
Apart from the high rotational speed, high demands are placed on the drive and the bearings of a spinning rotor due to the high risk of soiling from fibre residues. In the case of soiling of this type it is possible that the magnetically mounted rotor can no longer be held in the hovering state by the control. Crashes may then result and therefore considerable damage to the bearing, drive or spinning rotor. However, if mechanical damage is already present or the bearing magnets are demagnetised, the hovering rotor may fall during operation.
In order to avoid the hovering rotor falling, various monitoring possibilities are known from the prior art to recognise soiling or faults during operation and to be able to switch off the magnetically mounted drive prior to a crash. International Patent Publication WO 01/17096 A1 discloses the possibility of recognising faults in the magnetic bearing during operation by the evaluation of a sensor. German Patent Publication DE 10 2005 032 184 A1 provides that during operation, the current in the actuator coils is evaluated to control the position in the axial direction and deviations of the bearing air gaps from predetermined limit values are inferred therefrom.
It is possible for soiling or faults of the magnetic bearing to already be present before the drive machine is run up to operating speed. It would be desirable to recognise such soiling or faults as soon as possible and not firstly when the machine is rotating. If faults in the bearing are only recognised during the run-up or during stationary operation, it is necessary for the machine to be reliably decelerated. This takes time and lowers the operating safety.